Add 3D beam 3-point flexion validation: SOFA vs FreeFEM

[Fimache]

Context

Adds a new SOFA vs FreeFEM cross-validation case: 3D three-point bending
of a rectangular-section beam (P1 tetrahedra). Complements the existing
3D validation cases (circular traction, circular bending, torsion,
distributed load).

Setup

• Geometry: rectangular beam, L=1.0, H=0.2, W=0.2 (beam3d_tet.msh).
• Boundary conditions, identified by exact node-coordinate matching:
  • Left support line (x=0, y=0): pin — ux = uy = uz = 0
  • Right support line (x=L, y=0): roller — uy = 0 only
  • Midspan load line (x=L/2, y=H): distributed line load along -y,
    split across the line's nodes using trapezoidal tributary-length
    weights so the total applied force equals P.

Contents

• sofa_beam3d_threept.py: SOFA scene — NewtonRaphsonSolver +
  SparseLDLSolver, LinearSmallStrainFEMForceField, pin/roller
  supports via FixedProjectiveConstraint /
  PartialFixedProjectiveConstraint, midspan load via
  ConstantForceField.
• freefem_beam3d_threept.edp: FreeFEM solve of the same problem
  (same geometry, same BCs), writes raw results to a plain-text file.
• comparaison_script3d_3pt.py: runs both solvers, pairs nodes
  between SOFA and FreeFEM by coordinates (with grid-snapping to avoid
  float-noise mismatches between the two mesh readers — see in-code
  comment), computes discrete RMS on ux/uy/uz, and reports midspan
  deflection (node nearest (L/2, H, W/2)) from both solvers.
• params_beam3d_threept.json: load and material parameters
  (P, Young's modulus, Poisson's ratio).
• beam3d_tet.msh: input mesh (kept — required to run the case,
  not a generated result).